1. Field of the Invention
The present invention relates to a direct color thermal printing method using a thermal color recording medium which is colored when heated. The present invention also relates to a direct color thermal printer.
2. Description of the Related Art
As a thermal color recording medium, there is known a direct color thermal recording medium (hereinafter simply called a thermal recording medium) disclosed, for example, in U.S. Pat. Nos. 4,734,704 and 4,833,488 (both corresponding to Japanese Laid-open Patent Application No. 61-213169), having thermosensitive coloring layers for yellow, magenta and cyan which are laminated or formed on a supporting material in this order from the top side of the medium. In this type of thermal recording medium, the heat sensitivities of the thermosensitive coloring layers become lower as the distance from the upper surface increases. Furthermore, the coloring layers have properties that each coloring layer is optically fixed by electromagnetic rays of a respective specific wave length range.
When recording a multi-color image on the above-described thermal color recording medium, a thermal head having a plurality of heating elements arranged in a line is used. First, a yellow frame of a multi-color image is thermally recorded in the coloring layer for yellow, or the first layer that is disposed on the uppermost of the coloring layers, while the thermal head is moved relative to the thermal color recording medium. After recording a yellow frame of the multi-color image in the first layer in this way, the thermal color recording medium is exposed to light having a wave length range by which a diazonium salt compound still contained in this first layer is decomposed. Thereby, the first layer is optically fixed by decomposing the diazonium salt compound that has a capacity for coupling. Next, a magenta frame of the multi-color image is recorded in the coloring layer for magenta, or the second layer that is disposed in the second position from the top side of the medium, by using a higher heat energy than that applied for the yellow frame recording. Thereafter, the second layer is optically fixed by being exposed to light having a wave length range that decomposes a diazonium salt compound still contained in the second layer and having a capacity for coupling. Then, the highest heat energy is applied to the thermal color recording medium, so as to record a cyan frame of the multi-color image in the coloring layer for cyan, that is, the third layer disposed at the bottom of the coloring layers. Finally, light having a wave length range that decomposes a diazonium salt compound still having a capacity for coupling, is applied to optically fix the third layer.
In a one-pass method, a multi-color image is thermally recorded by passing a thermal recording medium once through a path along which three thermal heads are mounted at a predetermined interval. With this one-pass method, although the recording time can be shortened, three thermal heads are driven substantially at the same timing. Therefore, it is necessary to use a power supply circuit for powering three thermal heads substantially at the same timing. It is also necessary to maintain constant the transport speed of a thermal recording medium for each color recording. Under such recording conditions, the necessary power to be supplied to three thermal heads becomes three times as large as a conventional one-head three-pass method, although the power supply period is short or in one recording cycle (one line recording). An important issue of reducing maximum instantaneous power consumption occurs, from the view point of reducing the size and cost of an apparatus.
Another issue to be considered is a paper feed speed. A paper feed speed v3 is determined depending on one cycle time necessary for the thermal recording of the cyan thermosensitive coloring layer or the third layer requiring the largest heat energy, and the other paper feed speeds v1 and v2 for the thermal recording of the yellow and magenta thermosensitive coloring layers are adjusted, based upon the paper feed speed V3. From this point of view, it is desired to provide a suitable control method capable of reducing heal loss of the thermal heads for recording the upper two coloring layers. Namely, for the thermal recording of the yellow and magenta thermosensitive coloring layers requiring smaller heat energy than the lowest thermosensitive coloring layer, the time period while the thermal heads are actually powered during one recording cycle can be shorter than the cyan thermosensitive coloring layer, assuming a constant head application voltage is applied. As a result, the cooling time with the thermal heads not being powered is made longer. Accordingly, there occurs a problem that the thermal heads are cooled to an unnecessarily lower temperature until the next thermal recording starts.
A digital or analog image signal is supplied to a color thermal printer for printing a multi-color image on a thermal recording medium as represented by the multi-color image. The amount of image data of a multi-color image is relatively large. It takes a lot of time for transferring such image data to be printed to a color thermal printer. Furthermore, the capacity of a buffer memory for temporarily storing transferred image data becomes relatively large.